1. Field of the Invention
The present invention relates to a Multiple-Input Multiple-Output (MIMO) wireless local area network (WLAN) system, and more particularly, to a method for selecting at least one antenna from a plurality of antennas and transmitting a data frame via the selected antenna or antennas.
2. Discussion of the Background
A WLAN is a short distance communication network where signals can be transmitted wirelessly between nodes, such as stations and an access point. The WLAN can improve the readiness and extensibility of a wired LAN, can reduce the cost of operating and maintaining an LAN, and can provide users with a more convenient network access environment.
Currently, the usage of various types of portable devices, such as portable digital assistants (PDAs), portable media players (PMPs), tablet personal computers (PCs), and the like is expanding. As the use of portable devices increases, users' demands for network connection functions using the portable devices are also increasing, thus making the WLAN an attractive option.
A representative standard associated with the WLAN is the Institute of Electrical and Electronics Engineers (IEEE) 802.11 Wireless Fidelity (Wi-Fi) standard. Since the IEEE 802.11 standard was initially released in 1997, the IEEE 802.11 standard has been the base of various extensions such as 11a, 11b, and 11g. In order to support higher performance, 802.11n is currently being standardized.
The IEEE 802.11n standard adopts an MIMO system configuration scheme to support high data rates in the physical layer. In the MIMO scheme, a transmitting end can transmit data via multiple transmitting antennas, the data may pass through various types of paths, and a receiving end can detect the data using the signals that are received from the various paths via multiple receiving antennas. Through this, the MIMO scheme can improve data rates and reduce interference that may occur in a multi-path environment.
Therefore, in an IEEE 802.11n WLAN environment, stations and an access point each have multiple antennas. Through the above configuration, in comparison to existing versions, it is possible to support an improved data rate in the physical layer.
The Korean Patent Application No. 10-2006-97537 relates to a MIMO WLAN system. FIG. 1 illustrates a general configuration of the MIMO WLAN system described in the above application. Referring to FIG. 1, the MIMO WLAN system includes multiple stations 110 and 120, and an access point 130 that receives data from the stations 110 and 120, which is the same as a general WLAN system.
The difference between the MIMO WLAN system and the general WLAN system is that in the MIMO WLAN system, each station and access point includes multiple antennas. Thus, the first station 110 includes multiple transmitting antennas 111 and 112, the second station 120 includes multiple transmitting antennas 121 and 122, and the access point 130 includes multiple receiving antennas 131, 132, 133, and 134. The stations 110 and 120 can respectively transmit data via the transmitting antennas 111, 112, 121, and 122 through different paths. The access point 130 may receive signals transmitted through different paths via the receiving antennas 131, 132, 133, and 134.
Unlike the arrangement shown in FIG. 1, the general WLAN system may be arranged such that each station includes two transmitting antennas and an access point also includes only two receiving antennas. Thus, each station transmits data via the two transmitting antennas, and the access point receives the data from each station via the two receiving antennas. If the two stations simultaneously transmit data, four data streams can reach the access point simultaneously. In this case, a data collision may occur and the access point's two receiving antennas may not be able to discriminate among the four data streams. As described above, in the general WLAN system, if the receiving end includes a fewer number of antennas than a number of independent data streams from the transmitting end, the receiving performance may be significantly degraded.
Accordingly, a new technology is proposed to improve the data throughput by adopting a selective diversity scheme to multiple antennas.